The Bad Wolf Process was a process used in the Hesguard Institute in the 53rd century. It was used to purge negative emotions such as anger, fear, and hate, and put them in a Sin-Eater where they would be dispersed. This process did not work, and often those who had undergone it would experience psychosis and go on to commit worse atrocities. (COMIC: Sin Eaters)
When then Ninth Doctor underwent the procedure, his Sin-Eater became sentient and was an algamation of his second, fifth, seventh, eighth, and ninth faces, as well as the face of the War Doctor. (COMIC: Sin-Eaters).
The Doctor stated that Sin-Eaters of any telepathic individuals would be sentient. (COMIC: Sin-Eaters).